Recently, R&D of a small-sized X-ray generator utilizing laser Compton scattering has been watched. Here, laser Compton scattering is that radiation rays like X-rays are generated at collision of laser and electron beam. In order to perform laser Compton scattering, it is required to produce very high pulse-strength of laser and high luminance of electron beam. However, the production of high pulse-strength of laser has been very difficult as described below. On the other hand, it has been known that high-luminance of electron beam can be produced by circular-accelerators such as synchrotron and cyclotron. So that, it has been presented the conventional apparatus in which laser oscillators are set in the electron beam loop-path of the circular-accelerators. However, the circular-accelerators are very big, usually several kilometers in peripheral length, so that, the above method has been unsuitable for industrial uses.
Circular-accelerators can generate high-luminance of coherent X-rays in the energy range from several keV to 100 keV. But, such accelerators will be never utilized for industrial usages due to their huge size. However, small-sized alternatives to produce X-rays as strong as synchrotron X-rays have been scarcely known.
To generate laser, fiber laser amplifiers and optical resonators have been known. Generation of laser by means of the fiber laser amplifiers is principally based on the induced emission which is usually conducted by irradiation of exciting laser onto the optical fibers doped with induced emission materials, so that, the more fiber becomes long, the more laser becomes strong. However, it is difficult to strengthen laser because oscillation state is easily disappeared due to thermal expansion of optical fibers.
Patent Literature 1 discloses the multi-fiber laser amplifier in which transmission of pumping energy is blocked by an attenuator (optical isolator) inserted between parallel pumping step and backward pumping step. However, this type of fiber laser cannot much raise laser strength due to thermal expansion of optical fiber.
Patent Literature 2 and Non-Patent Literatures 1 and 2 disclose the higher harmonic wave mode-lock fiber laser oscillator which comprises an optical loop consisting of fiber laser amplifier, GHz-driving-LiNbO3-modulator, Fabry-Perot filter, band pass filter and Mach-Zehnder optical modulator, and the like, and the recovery mode-lock fiber laser oscillator which comprises an branching optical loop including photo-coupler, RF power amplifier and Mach-Zehnder optical modulator, and the like, for the purpose of optical communication laser oscillator. Modern times, small-sized fiber laser oscillator (10 dB, 10 times amp.) with ca.10 m-long fiver and large-sized fiber laser oscillator (40 dB, 10,000 times amp.) with ca.10,000 m-long fiber which are similar to the above ring-fiber oscillator are commercially available for the purpose of optical communication laser oscillation. Such long ring-fiver oscillators are easily lost oscillation state due to thermal expansion. The above higher harmonic wave mode-lock fiber laser oscillator and recovery mode-lock fiber laser oscillator disclosed by Patent Literature 2 and Non-Patent Literatures 1 and 2 have been invented to adjust deviance between fundamental frequency and modulated frequency due to thermal expansion. However, pulse-strength of laser generated by the oscillators was only several pico-joules (10−12 joules) as described in Non-Patent Literature 2. From this, it will be noted that the hitherto-known fiber laser oscillators are for the purpose of increasing transmission rate of communication signals but not for the purpose of generating high-strength of laser.
On the other hand, an optical resonator has been known as a tool to amplify laser. Laser-amplification by the optical resonator is made by laser interference on the resonator mirror surfaces, so that, the amplification depends on reflectance of the resonant mirrors. As the optical resonators, Fabry-Perot ring-resonator, Michelson interferometer-typed resonator and Fox-Smith interferometer-typed resonator, etc. have been known.
Laser-amplification only occurs under the condition that a resonator length is equal to an integral multiple of a half wave-length of laser. This is so-called a stationary wave standing. The resonance width of a stationary wave is determined by reflectance of resonator mirrors. When intend to obtain high gains, the more the reflectance of mirrors becomes high, the more the resonance width becomes narrow. For example, when suppose a resonator for obtaining 1000 times in gain using a mirror with a reflectance of 99.9%, the resonance width is to be 24 kHz or about 1 Å in resonance position. Consequently the resonance state must be easily disappeared by environmental disturbance of thermal expansion and vibrations. In order to maintain the resonance state, extremely precise feedback-regulations using piezoelectric driving of the resonator mirrors is required, so that, laser-amplification of the conventional optical resonators is limited to about 1000 times due to limitations of mechanical regulations.
Many laser-amplifying apparatuses using optical resonators have been presented (Patent Literatures 3-7 and Non-Patent Literature 3).
Non-Patent Literature 3 discloses the optical resonator to generate single-frequency laser pulses using the Fox-Smith interferometer-typed optical resonators embedding concave mirrors and piezoelectric device for mechanically controlling resonant mirrors. The laser powers generated by this method have been reported to be at most 15 mW.
Patent Literature 3 discloses the simple-structured laser oscillators for encoding electrical signals onto optical beams in which the Fabry-Perot resonators having laser cavities filled with rare-earth doped optical fibers for optical communication and the Fox-Smith interferometer-typed optical resonators having reflecting mirrors. The purpose of these laser oscillators is to provide fine optical conveying waves with longitudinal mode selections but not to generate lasers having high pulse strength. The pulse strength of this typed laser oscillators was at most to micro joule levels due to declination in oscillation width by thermal vibrations, even if raising oscillation powers.
Patent Literature 4 discloses the optical resonator to generate laser light by irradiating pumping-light (exciting laser) onto solid-state lasers, wherein the pumping light is generated by injecting currents into the laser diode or solid-state laser (induced emission medium) which is embedded in the inside of the resonator. This method is a convenient generation method using inexpensive and small-sized laser diodes, however, cannot generate laser light to conduct laser Compton scattering, because of low amplification as explained above.
Patent Literature 5 discloses the laser amplifier to pump lasers using diodes. This amplifier is an apparatus to focalize laser beams into the medium by thermal lens which are put in the inside of the resonator, wherein the resonator is embedding a laser-active-solid-state medium. However, this method cannot generate laser beam to conduct laser Compton scattering, because of low amplification as explained above.
Patent Literature 6 discloses the apparatus to generate laser light using a giant mode-locked laser oscillator and optical resonator. But the giant mode-locked laser oscillator is a very expensive huge apparatus, requires extremely high level of feedback regulations, and is limited to at most 1000 times in gain, therefore, the pulse strength of the laser light generable by this method is at most 100 μJ.
Patent Literature 7 discloses the multistage amplification-typed laser system having multiple resonators placed in series for the purpose of semiconductor exposure. This typed optical resonator is the one to raise gradually the amplification of laser light by subsequent transmission of laser light. The amplification of laser light is limited by mechanical regulative accuracies of resonance width. Even if the system is intended to raise the laser amplification up to the gains enough for laser Compton scattering, the apparatus requires many resonators interconnected in series and each optical resonator requires extremely high level of regulation systems of resonance width. Therefore, it is in need impossible to use such a multistage laser system having multiple resonators placed in series for the purpose of laser source for laser Compton scattering.
The generation of great strength of laser is in principle possible by the combination of a giant exciting laser source and giant RF-oscillator to produce great powers, however, such a giant combination system is unsuitable for industrial usages.
The conventional optical resonators as described above are able to produce laser with low amplification but cannot generate polarized laser.
Several apparatuses to generate Compton scattering X-rays have been presented (Patent Literatures 8-10).
Patent Literature 8 discloses the apparatus to generate X-rays by collision between laser and electron beam in the inside of the Fox-Smith interferometer-typed resonator having a laser oscillator between a pair of mirrors which is set in the electron beam loop-path of the circular-accelerators. Because laser is provided only by the laser oscillator, the amplification of the laser beam supplied by the laser oscillator is limited to at most 1000 times in gain as explained above, even if reflectance of the reflecting mirrors is much raised. Therefore, it is difficult to generate strong Compton scattering X-rays by this apparatus.
Patent Literature 9 discloses the apparatus to generate short-wavelength light by collision between mode-locked laser and electron beam in the inside of the optical resonator providing a unit of multi concave mirrors arranged with a pair of concave mirrors in series, wherein the laser beam is in repetition reflected and focused between the concave mirrors and the collision of the laser and electron beam is carried out in the focused region of the laser beam. Because this apparatus in which the mode-locked laser is merely repeat-reflected between a pair of concave mirrors is, in structure, the same as the Fox-Smith interferometer-typed optical resonator, the amplification of the laser produced by this apparatus is limited to at most 1000 times in gain as described above. Therefore, this apparatus may generate short-wavelength light for a photolithography usage, but cannot generate strong laser Compton scattering X-rays.
Patent Literature 10 discloses the apparatus to generate X-rays or γ-rays by collision between laser and electron beam in the inside of the Fox-Smith interferometer-typed optical resonator providing a pair of mirrors with super reflectance which is set in the electron beam loop-path of the circular-accelerators. Also, the invention discloses the apparatus providing a set of the resonators aligning in parallel on the electron beam orbit. However, the optical resonator used in this apparatus is the conventional resonator providing a pair of concave mirrors. Even if the mirrors with 99.99984% in reflectance can be used, the amplification of laser beam is limited at most 1000 times as explained above. Therefore, this apparatus cannot generate strong laser Compton scattering X-rays.
For the purpose of the development of optical resonators to generate laser with 1 mJ or greater in pulse strength, the development of laser-resistant resonant mirrors might be challenged. It has been known the synthetic optical quartz glass for a semiconductor exposure usage (Patent Literature 11), the highly purified silica glass material with low refraction index (Patent Literature 12), the synthetic quartz glass (Patent Literature 13), the optical quartz glass for a excimer laser usage (Patent Literature 14), the laminated metal coat for a excimer laser usage (Patent Literature 15), the dielectric multi coat consisting of high-refraction tantalum oxide thin layers and low-refraction silica thin layers (Patent Literature 16) and the ceramic materials such as sapphire (Patent Literature 17), etc. In addition, it has been that the reflecting mirrors deposited with multilayer structures containing diamond layers having high thermal conductivities can be used for optical devices such as semiconductor lasers (Patent Literature 18).
However, the present inventors noticed that most of the above resonant mirrors and reflecting mirrors were broken by exposure of laser light with 300 μJ in pulse strength.
In view of the above described circumstances, the present inventors have found that outstanding laser amplification exceeding 3,000 times is achieved by self-oscillation of the optical loop-path which is formed by loop-connecting an optical resonator and fiber laser amplifier with an adjusting cable (Non-Patent Literature 4), and based on this finding have invented an innovative laser oscillator (Patent Literature 19). Using our laser oscillator, it has been possible to generate pulse laser with pulse strength of 300 μJ or more. From this, we considered that the problem of laser amplification has been no longer remained. But, after that, we noted that our laser oscillator was difficult to produce laser with pulse strength of 1 mJ or more due to saturation in amplification.
As described above, most of the laser amplifiers such as optical resonators and fiber laser oscillators have been used for optical communication and laser processing, but small-sized laser generators to produce laser which is strong enough to radiate laser Compton scattering X-rays has been scarcely known.